There have been many examples of projection systems for displaying images including video images. For example, such projection systems, and components thereof, are described in U.S. Pat. Nos. 5,313,321; 5,307,186; 5,303,403; 5,300,942; 5,260,815; 5,245,453; 5,231,432; 5,189,534; 5,085,506; 5,085,498; 5,032,924; 5,012,274; 4,994,901; 4,969,734; 4,848,879; 4,818,098; 4,751,509; and 4,675,702, which are each incorporated herein by reference.
Generally, such projection systems disclosed in the foregoing patents, include liquid crystal display (LCD) panels for producing color images. The colored light for forming the color images can be generated by using color filters to separate the desired colors from a white light source, such as an incandescent light source. Alternatively, the desired colors can be obtained from a white light source by passing the white light through a series of dichroic devices such as dichroic mirrors, thereby eliminating unwanted light. The filtered colored light is then modulated by LCD panels.
The use of devices such as color filters and mirrors results in the loss of light intensity. Thus, the overall efficiency of the system, where efficiency is measured as lumens per watt, is impaired.
Additionally, the LCD panels inherently cause light to be lost. In this regard, an LCD panel includes an alignment layer, which cooperates with the liquid crystal layer for permitting polarized light to enter the liquid crystal layer. Therefore, polarizers are used with the LCD panels to polarize the light entering the alignment layers. Such polarizers necessarily block all light except the desired polarized component. Thus, even more light is lost due to the polarizers, thereby reducing the overall brightness of the final image emitted from the panel.
In addition to the light lost due to the use of filters and polarizers, the aperture ratio of the LCD panels can further contribute to the amount of light lost in the system. Thus, due to such light loss factors, only a small fraction of the initial amount of light provided by the light source is utilized in the formulation of the output image.
While such image projection systems may adequately project full color images in low ambient light conditions, they do not always perform satisfactorily in high ambient light conditions for some applications. In this regard, due to the lower intensity of the resulting image, it must, by necessity, be viewed in a darkened room. This is not always an acceptable viewing condition. As image projection systems are often used in conjunction with other activities which require bright ambient light, such as note taking, it is desirable to have an image projection system, which is capable of producing a bright image, even in bright ambient light conditions.
In an attempt to provide a bright full color image, projection systems have employed laser illumination as described in U.S. Pat. Nos. 5,287,096; 5,272,473; 5,214,420; 5,214,419; 5,206,629; 5,192,946; 5,170,156; 5,162,787; 5,128,660; and 5,079,544, which are incorporated herein by reference. In the foregoing laser illumination patents, an image projection system includes three discrete colored lasers, one red, one green, and one blue. The colored lights emanating from the lasers are combined to form a white light, which, in turn, is directed onto a single spatial light modulator to produce a full color image.
However, in order to achieve full color image projection, the three lasers of the aforementioned patents are activated sequentially to produce alternatingly three colored images. For example, the red laser is first activated and deactivated, and then the green laser is activated and deactivated. Finally, the blue laser is activated and deactivated before repeating the cycle. The total activation and deactivation cycle time for the three lasers is set to be less than the critical flicker frequency of the human eye. In this manner, a red, a green, and a blue image will appear to coalesce into a single full color image in the eye of the viewer.
Alternatively, a full color image can be produced by sequencing combinations of the three lasers simultaneously. For each pixel, the proportions of the contribution of each laser would be adjusted to produce a desired color resulting from the light emitted by the combination of lasers. For example, all three lasers could be activated momentarily simultaneously to emit a specific amount of colored light to achieve the overall desired color for a given pixel. The lasers are then extinguished, and then activated selectively to emit another color combination for the next pixel. In this manner, all of the pixels requiring different colors are illuminated sequentially. To the human eye, it would appear that all of the different colored pixels have combined to form a single full color image.
While the patented laser projection systems may be capable of producing relatively bright full color projection images, they require that the lasers be interrupted sequentially in order to produce various colored images. As a result of the interruption of the lasers, the amount of light produced by each laser is diminished or even not activated at all, during the modulation procedure. Thus, the resulting colored image does not fully utilize the intensity of the lasers for illumination purposes at any one time.
For example, where the lasers are alternately activated and deactivated in repeating sequences (e.g., red, green, blue, red, green, blue, etc.), each laser may only be activated for one third of one cycle. As a result, each laser produces only a fraction of the amount of light that it is potentially capable of producing at that given instant of time, and the other two lasers are totally extinguished. The resulting full color image projected by the patented projection system is produced in an inefficient manner for a relatively low energy cost per lumen of output. Only a fraction of the laser light is utilized during the modulation procedure.
In short, it would be highly desirable to have a new and improved projection system which can generate a bright image in a highly efficient manner for a relatively low energy cost per lumen of output. In this regard, for a given size light source, an extremely bright image should be created, and yet the overall size of the projector should be small and compact. Thus, the projection system should be highly efficient, and relatively inexpensive to manufacture.
Therefore, it would be highly desirable to have a new and improved image projection system which can produce a bright image, even a full color image. Such an image projection system should operate in a highly efficient manner and should be relatively inexpensive to manufacture.